The present invention relates to an optical disc apparatus for use in recording and/or reproducing data on and/or from an optical disc. Particularly, this invention relates to an optical disc apparatus in which an optical pickup is stably moved to a target track and an optical-pickup movement control method to be installed in such an optical disc apparatus.
In optical disc apparatuses, data recording or reproduction is performed in such a manner that a light beam (a laser beam) for recording or reproduction is emitted from an optical pickup to an optical disc having a spiral or concentric tracks formed thereon, with a light spot moving along the tracks. The light spot is formed by converging the light beam through an objective lens attached to the optical pickup and required to be in focus while moving along the tracks. For this purpose, the objective lens is always under focus and tracking control.
One requirement for optical disc apparatuses is that an optical pickup be moved to a predetermined track or a target track designated by user operation in start-up, just before recording or during reproduction. This operation requires: focus control under which the optical pickup is moved in the radius direction of an optical disc in a short time and stopped over a target track; and tracking control to the target track.
Such an operation in which an optical pickup is moved to a target track at a high speed and stopped over the target track is called multitrack jump.
Described below is an optical-pickup movement control technique for multitrack jump.
A sign-wave tracking error signal is generated whenever a light beam crosses a track while an optical pickup is moving toward a target track. A pulse is then generated at a rising moment at a zero crossing point (level zero) of each tracking error signal. This pulse is called a track crossing pulse. The number of track crossing pulses corresponds to the number of tracks the light beam has crossed. The number of track crossing pulses also indicates how much the optical pickup is moved in the radius direction of an optical disc. The reciprocal of a period of a track crossing pulse (or an interval between two consecutive track crossing pulses) indicates a moving speed of the optical pickup. The moving speed is controlled based on the reciprocal mentioned above in such a way that the speed is lowered as the optical pickup is moving closer to a target track and the then optical pickup is stopped over the target track.
Discussed below are disadvantages of the optical-pickup movement control technique described above.
Tracking error signals are generated based on reproduced signals gained from light beams reflected from an optical disc. The tracking error signals having sign waves could suffer decrease in signal level, phase shift or signal drop-outs, etc., when the optical disc has a damaged or dirty section on its surface. This abnormal condition in which signals are not reproduced normally is called a defect of reproduced signals. Such a defect of reproduced signals causes erroneous detection of a moving distance of an optical pickup (the number of tracks the pickup has crossed). This results in that the optical pickup cannot be stopped over a target track.
Japanese Unexamined Patent Publication Nos. 05(1993)-114145 and 05(1993)-166201 disclose techniques to solve such problems with compensation of dropped-out track crossing pulses with interpolated dummy pulses.
These techniques with interpolation of dummy pulses, however, still have a difficulty in compensation for variation in moving speed of an optical pickup for the period in which no track crossing pulses are gained, thus having a difficulty in adjustments to moving speed. In detail, the moving speed cannot be quickly adjusted when a defect of reproduced signals occurs. Particularly, when such a defect occurs just before a target track, the optical pickup cannot be stopped over this track. In other words, a so-called “off track” occurs.